As compared with widely used amorphous silicon (a-Si), amorphous (non-crystalline) oxide semiconductors have high carrier mobility (also called as field-effect mobility, which may hereinafter be referred to simply as “mobility”), wide optical band gap, and film formability at low temperatures, and therefore, have highly been expected to be applied for next generation displays which are required to have large sizes, high resolution, and high-speed drives; resin substrates having low heat resistance; and others.
In the oxide semiconductors, amorphous oxide semiconductors consisting of indium, gallium, zinc and oxygen (In—Ga—Zn—O, which may hereinafter be referred to as “IGZO”) have preferably been used. For example, non-patent literature documents 1 and 2 disclose thin film transistors (TFTs) in which a thin film of an oxide semiconductor having an In:Ga:Zn ratio equal to 1.1:1.1:0.9 (in atomic % ratio) was used for a semiconductor layer (an active layer). In addition, patent document 1 discloses an amorphous oxide semiconductor (IGZO) consisting of In, Ga, Zn, and O.
In patent document 2, amorphous oxide semiconductors consisting of In, Zn, Sn, and O (In—Zn—Sn—O, which may hereinafter be referred to as “IZTO”) have been used.
Advancement of display devices in recent years to have large size, high definition, and high-speed drive necessitates materials having superior properties. Specifically, when an oxide semiconductor is used as a semiconductor layer of a thin film transistor, the oxide semiconductor is required to have not only high carrier mobility but also excellent TFT switching characteristics (transistor characteristics or TFT characteristics). More specifically, the oxide semiconductor is required to have, for example, (1) a high on-state current (i.e., the maximum drain current when a positive voltage is applied to both a gate electrode and a drain electrode); (2) a low off-state current (i.e., a drain current when a negative voltage is applied to the gate electrode and a positive voltage is applied to the drain voltage, respectively); (3) a low S value (Subthreshold Swing, i.e., a gate voltage needed to increase the drain current by one digit); (4) a stable threshold value (i.e., a voltage at which the drain current starts to flow when a positive voltage is applied to the drain electrode and either a positive voltage or a negative voltage is applied to the gate voltage, whose voltage may also be called as a threshold voltage) showing no change with time (which means that the threshold voltage is uniform in the substrate surface); and (5) a high mobility.
Furthermore, TFTs using the oxide semiconductor layers are required to have excellent resistance to stresses such as voltage application and light irradiation (stress resistance). It is pointed out that, for example, when a voltage is continuously applied to the gate electrode or when light in a blue-emitting band in which light absorption arises is continuously irradiated, charges are trapped at the interface between the gate insulator layer and the semiconductor layer, resulting in a variation of switching characteristics, such as a shift of the threshold voltage of a thin film transistor. In addition, for example, when a liquid crystal panel is driven or when a negative bias is applied to the gate electrode to turn on a pixel, the TFT is irradiated with light leaked out of the liquid crystal cell, and this light puts stress onto the TFT to cause deterioration in the characteristics. Indeed, when a thin film transistor is used, a variation of switching characteristics due to stress by voltage application causes a lowering of reliability in a display devices itself, such as a liquid crystal display or an organic EL display. For example, a variation of switching characteristics in an organic EL display creates a need to flow a current of several μA or higher for driving an organic EL element. Therefore, an improvement in the stress resistance (a small variation before and after the stress tests) has eagerly been desired. In particular, as display devices become large in size and high-speed drive, higher mobility and improved stress resistance are liable to be required.
It has been known that the deterioration of TFT characteristics by stresses such as bias application and light irradiation is caused by the formation of, for example, defects in the oxide semiconductor itself, and defects at an interface between the oxide semiconductor and a passivation film formed to protect the oxide semiconductor surface. It has been also known that the deterioration of TFT characteristics is caused by the formation of defects at an interface between the oxide semiconductor and an etch stopper layer which is formed on a surface of the oxide semiconductor to prevent damages during an etching process of a source-drain electrode. As the passivation film and the etch stopper layer, oxide films such as SiO2, Al2O3, and HfO2 are widely used. However, if water and oxygen molecules are adsorbed on the surface of the oxide semiconductor layer including the interface between the semiconductor layer and the passivation film or the etch stopper layer, then carrier concentration in the semiconductor layer is either decreased or increased, resulting in a shift of the threshold voltage and deterioration of reliability.
In order to keep up with the advancement in display devices in recent years to large size and high-speed drive, a material superior in TFT characteristics and stress resistance has thus been highly required.